Remove an assertion that there was a call to pause content between two
content -> ad transitions.
Also, only use the player position for resuming an ad on reattaching if the
player is currently playing an ad, in case IMA pauses content before the player
actually transitions to an ad.
Issue: #3430
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=176365842
